The present invention relates to a semiconductor memory device and more particularly to a flash memory device.
Semiconductor memories are vital components in digital logic systems, such as computers and consumer electronics. Therefore, advances in the fabrication of semiconductor memories including process enhancements and technology developments through device scaling to higher densities and faster operating speeds improve the overall performance of digital logic systems.
Semiconductor memory devices may be characterized as volatile memory devices, such as Random Access Memory (RAM) devices, or non-volatile memory devices. In RAM devices, digital data is stored by either setting up the logic state of a bi-stable flip-flop in the case of static random access memory (SRAM) devices, or by charging/discharging a capacitor in the case of a dynamic random access memory (DRAM) device. In either case, the data is retained in memory only so long as the power is applied to the device. However, once power is no longer applied, stored data is lost from volatile memories.
In contrast, Non-volatile memories, such as Mask Read-Only Memory (MROM), Programmable Read-Only Memory (PROM), Erasable Programmable Read-Only Memory (EPROM), and Electrically Erasable Programmable Read-Only Memory (EEPROM), are capable of maintaining stored data in the absence of applied power. The non-volatile memory data storage mode may be permanent or reprogrammable depending upon the fabrication technology used to implement the device.
A combination of volatile and non-volatile operating modes are available in certain hybrid devices such as the non-volatile SRAM (nvSRAM). These memory devices are particularly well suited for use in systems that require a fast, programmable, non-volatile data storage capability. In addition, dozens of special memory architectures have evolved which contain some additional logic circuitry to optimize their performance of memory devices for application-specific tasks.
The incorporation and use of MROM, PROM, and EPROM devices in contemporary applications have proved difficult given the unique conditions necessary to erase and/or data within these devices. On the other hand, the EEPROM is capable of being electrically erased or written to, and as such has been successfully incorporated into many products. Indeed, the application of EEPROMs (e.g., flash memory) has widened recently to include auxiliary memories or system programming memories requiring continuous updates. In particular, so-called flash memory exhibits a higher degree of integration than other types of EEPROM and is thus advantageous in the implementation of large auxiliary memories.